A critical component of exercise devices used to increase the strength of the user, may be devices used to provide a variation in resistance. As some individuals are stronger than others, it may be useful to be able to vary the resistance from user to user, or of the same user over time to adapt to increases in strength. To develop strength, the muscle may be forced to move under tension. This tension may be provided by weights or hydraulics. Low inertia systems may also be used. These may include springs or other elastic components, or compressed air as forms of resistance. One advantage to low inertia systems, is high-speed work may be done without the interference provided by high mass of the resistance.
Regardless of the type of resistance used, the ability to change resistance may be considered a critical element to successful strength training. As a user becomes stronger, that user will desire a greater resistance. In addition, no two users are exactly the same in their strength potentials. Therefore, it may be desirable to have a system which allows for a great deal of variability in the resistance. It may also be desirable to be able to change the resistance easily. For some people changing heavy weight plates may be difficult or even impossible depending upon that person's strength. Particularly in any rehabilitation setting, it may be desirable to be able to change resistance with minimal effort. A system that includes rotating one or more knobs may require much less physical exertion to change the resistance as compared to lifting heavy weights to add or remove from a bar or other apparatus. If a user has limited physical ability and cannot change the resistance easily, that may preclude them from using the exercise device altogether. No exercise device is of any value if it is not used.